The effects of polyethylene glycol (PEG) on the solution properties of calf brain tubulin were investigated at pH 7.0. In vitro reconstitution is promoted by PEG with the polymers of higher molecular weight being more efficient in lowering the free energy of the propagation step of microtubule formation. The dependence of the apparent association constant of microtubule formation on PEG concentration was analyzed by the linked function theory of Wyman (Wyman, J., (1964) Adv. Protein Chem. 19, 224-286), leading to a conclusion that the thermodynamic instability of the system is reduced by formation of microtubules. Such conclusion was substantiated by the results of investigation of the preferential solvent interaction of PEG with tubulin by density measurements. Application of multicomponent thermodynamic theory shows that tubulin is preferentially hydrated in all PEG solutions leading to an increase in the chemical potential of PEG. This unfavorable thermodynamic interaction leads to phase separation as evidenced by the precipitation of tubulin at higher PEG concentrations. Concomitant monitoring of the conformation of tubulin by comparing the accessibility of sulfhydryl groups and circular dichroic spectra at pH 7.0 indicates that PEG does not induce observable structural changes in tubulin. The results of spectrophotometric titration of tyrosine residues are consistent with that of circular dichroic spectroscopic study that PEG prevents the protein from unfolding at pH 10.